71 lines
4.6 KiB
HTML
71 lines
4.6 KiB
HTML
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<title>std::nullopt_t</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_nullopt_t skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>nullopt_t</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-optional.html"><optional></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> nullopt_t<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p><code>std::nullopt_t</code> 是空类类型，用于指示 <code>optional</code> 类型拥有未初始化状态。特别是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a></span></span> 有一个以 <code>nullopt_t</code> 为单参数的构造函数，它创建不含值的 <code>optional</code> 。</p>
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<p><code>std::nullopt_t</code> 必须是非聚合<a href="cpp-named_req-LiteralType.html"><span style="font-style:italic">字面类型</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LiteralType)</span></a> ，而且不能有默认构造函数或 initializer_list 构造函数。</p>
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<p>它必须有一个接收实现定义字面类型的 <code>constexpr</code> 构造函数。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>nullopt_t</code> 构造函数上的制约，在于支持 <span class="t-c"><span class="mw-geshi cpp source-cpp">op <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">op <span class="sy1">=</span> nullopt<span class="sy4">;</span></span></span> 两者都是令 <code>optional</code> 不含对象的语法。</p>
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<p>此类的一个可能实现是</p>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> nullopt_t <span class="br0">{</span>
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<span class="kw1">explicit</span> <span class="kw4">constexpr</span> nullopt_t<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div><a href="cpp-utility-optional-nullopt.html"><span class="t-lines"><span>nullopt</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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<td><code>nullopt_t</code> 类型对象<br />
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<span class="t-mark">(常量)</span></td>
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